Add limit-container-width to Jira create branch

Since the page does not have a sidebar navigation,
it makes sense to limit the width of the container
instead of using the `fluid` option to avoid the form
stretching too much.

Changelog: changed

app/views/jira_connect/branches/new.html.haml

 - @hide_breadcrumbs = true
 - @hide_top_links = true
+- @content_class = 'limit-container-width'
 - page_title _('New branch')
 
 .js-jira-connect-create-branch{ data: @new_branch_data }