Add documentation: Overview of Frontend Testing

Closes #45732

See merge request gitlab-org/gitlab-ce!18571

Adjust SQL and transaction Prometheus buckets

See merge request gitlab-org/gitlab-ce!19650

Resolve "Add `/confidential` quick action for issues"

Closes #47145

See merge request gitlab-org/gitlab-ce!19615

Resolve "[Rails5] `StorageHelper#storage_counter uses commas as thousands separator` fails"

Closes #47375

See merge request gitlab-org/gitlab-ce!19725

Documentation does not need an exception to be picked

See merge request gitlab-org/gitlab-ce!19741

Updates eslint vue plugin version

See merge request gitlab-org/gitlab-ce!19648

i18n: externalize strings from 'app/views/award_emoji'

See merge request gitlab-org/gitlab-ce!19690

Resolve "Table on Failed Jobs tab is overflowing in IE11 and sm screens"

Closes #47679

See merge request gitlab-org/gitlab-ce!19743

Add Web IDE CI logs and MR switching docs

See merge request gitlab-org/gitlab-ce!19723

* master: (27 commits)
  Resolve "Bug: When creating an account with invalid characters the error is "Username already taken" not "Invalid characters used""
  Resolve "Provide ability to retrieve `visibility` level via Snippets API"
  Resolve "Quick actions are case sensitive"
  Image Diff Viewing + Download Diff Viewing
  Link to release post for group issue board docs
  moved strings into constants file
  Typo fix
  fixed condition check
  Loosen the 'newly created MR' matcher
  Ensure we look into the correct setion only when expanding a settings' section
  Rails5 fix expected `issuable.reload.updated_at` to have changed
  [Rails5] Pass class references instead of strings to middleware builder
  Sidebar Milestone - Fix wrong URL when selecting a parent group milestone
  Fix Banzai reference for milestones belonging to parent groups
  apply feedback
  fixed eslint
  added specs for is-active class added spec for openPendingTab in component
  more karma fixes
  karma updates
  Override exclusive_lease_key method in RecordsUpload
  ...

Resolve "Bug: When creating an account with invalid characters the error is "Username already taken" not "Invalid characters used""

Closes #45575

See merge request gitlab-org/gitlab-ce!19610

Resolve "Bug: When creating an account with invalid characters the error is "Username already taken" not "Invalid characters used""

Resolve "Provide ability to retrieve `visibility` level via Snippets API"

Closes #35158

See merge request gitlab-org/gitlab-ce!19620

Resolve "Quick actions are case sensitive"

Closes #47050

See merge request gitlab-org/gitlab-ce!19614

Add active class to active file in IDE commit panel

Closes #46051

See merge request gitlab-org/gitlab-ce!19646

Resolve "Regressions for assigning group milestone from ancestor"

Closes #47735

See merge request gitlab-org/gitlab-ce!19721

Merge branch '47513-upload-migration-lease-key-is-incorrect-for-non-mounted-uploaders' into 'master'

Resolve "Upload migration lease key is incorrect for non-mounted uploaders"

Closes #47513

See merge request gitlab-org/gitlab-ce!19600

Image Diff Viewing + Download Diff Viewing

See merge request gitlab-org/gitlab-ce!18768

Link to release post for group issue board docs

See merge request gitlab-org/gitlab-ce!19677

Resolve two QA failures

Closes gitlab-qa#271

See merge request gitlab-org/gitlab-ce!19655

Fixed IDE jobs empty state showing when loading

Closes #47766

See merge request gitlab-org/gitlab-ce!19706

Typo fix

See merge request gitlab-org/gitlab-ce!19720

Rails5 fix expected `issuable.reload.updated_at` to have changed

Closes #47366

See merge request gitlab-org/gitlab-ce!19733

Since rails 5 beta 2 support of petabytes and exabytes were added to the
`number_to_human*` methods.
See https://github.com/rails/rails/pull/22759

So for rails5 the comma separator gets shown for big numbers (EBs).