Override the `ApplicationSetting` default visibility_level setters so
they accept strings & integers for the levels.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

We dont use them anywhere, so why have them around?

Add `visibility` & `visibility=` methods to the
`Gitlab::VisibilityLevel` module so the `visibility_level` can be
get/set with a string value.

Use strings for the ApplicationSetting properties:
 - restricted_visibility_levels
 - default_project_visibility
 - default_snippet_visibility
 - default_group_visibility

Fixed the position of the loading spinner in the issue template dropdown
toggle. It was previously just positioned with the content, this now
correctly places it in the same place the chevron down is

Closes #28906

It was taking up space when it didn't really need to be taking up space

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9245

Instead of exposing the VisibilityLevel as Integer, expose it as
String `visibility` for Project and ProjectSnippet.

Filter queries also accept the `visibility` as String instead of
`visibility_level` as Integer.

Also remove the `public` boolean.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The extra spacing at the top is caused the user-callout box that should be fixed in another merge request

 Rename column in the database
 Rename fields related to import/export feature
 Rename API endpoints
 Rename documentation links
 Rename the rest of occurrences in the code
 Replace the images that contain the words "build succeeds" and docs referencing to them
 Make sure pipeline is green and nothing is missing.

updated doc images

renamed only_allow_merge_if_build_succeeds in projects and fixed references

more updates

fix some spec failures

fix rubocop offences

fix v3 api spec

fix MR  specs

fixed issues with partials

fix MR spec

fix alignment

add missing v3 to v4 doc

wip - refactor v3 endpoints

fix specs

fix a few typos

fix project specs

copy entities fully to V3

fix  entity error

more fixes

fix failing specs

fixed missing entities in V3 API

remove comment

updated code based on feedback

typo

fix spec

Closes #26607

Closes #28844