Standardize recently added commit diff api url

Fixed up wording on the new SSH key page.

update gitlab:check to look for update hook, not post-receive

Use "/commits/:sha/diff" as opposed to "/commit/:sha", keeping in line
with existing api urls (e.g. "/projects/:id", etc.)

Fix 500 error resulting from a diff api call with an invalid commit hash

Move "/commits/:sha/blob" to "/blobs/:sha", leaving the old path for
backwards compatibility.

Add ability to get a single commit via "/commits/:sha"

Show permissions for all project.
Add ability to remove user from group if not an owner
Remove unnecessary admin controller

Provide UI with explanation what happens when you block or remove user
When remove user - remove all groups where user is an only owner

When block user - remove it from joined groups
Dont remove user from own resources (groups, users)

Fix project_id field in MergeRequest API

Clear redis cache and precompile assets during update to 6.0. Fix #4874 #4840

As a consequence on the namespaces change from 5.4 to 6.0, after upgrade
lots of links on the dashboard were not working (pointing to projects
without the group part on the url).
Also, after upgrading to 6.0-stable, the network (branch history) function
does not work. The progress bar rotates forever, but the network graph cannot
be generated.

Clearing redis cache fixes those issues.