This refactors the Project#protected_for? specs to separate them into
two contexts: when the ref is a full ref and when the ref is a ref name.

This reverts commit ec4730478b798270781257913ee4cede673d4d4e.

This implements Repository#ambiguous_ref? and checks if a ref is
ambiguous before trying to resolve the ref in Project#protected_for?

Reworks Project#resolve_ref to return Gitlab::Git::Branch,
Gitlab::Git::Tag or raise an AmbiguousRef error.

CE Backport: Epic issue list and related issue list re-design

See merge request gitlab-org/gitlab-ce!23658

Resolve "Navigating unresolved discussions on Merge Request page"

Closes #51122

See merge request gitlab-org/gitlab-ce!22789

Resolve "Extended user centric tooltips"

Closes #50157

See merge request gitlab-org/gitlab-ce!23231

[CE] Backport SAML unlink changes: UrlBlocker#ascii_only

See merge request gitlab-org/gitlab-ce!23627