Added option to disable migrations (#13114)

* Added option to disable migrations

This patch introduces DISABLE_MIGRATIONS parameter in [repository]
section of app.ini (by default set to false). If set to true
it blocks access to repository migration feature.

This mod hides also local repo import option in user editor if
local repo importing or migrations is disabled.

* Alter Example config

DISABLE_MIGRATIONS set to false in example config to
match its default value.

* HTTP error 403 instead of 500 on denied access to migration

* Parameter DISABLE_MIGRATIONS exposed via API

Fixes: 04b04cf854bcb3ed7659442bcf79822bdebe29e9
Author-Change-Id: IB#1105130

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -74,6 +74,7 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `DEFAULT_REPO_UNITS`: **repo.code,repo.releases,repo.issues,repo.pulls,repo.wiki,repo.projects**: Comma separated list of default repo units. Allowed values: \[repo.code, repo.releases, repo.issues, repo.pulls, repo.wiki, repo.projects\]. Note: Code and Releases can currently not be deactivated. If you specify default repo units you should still list them for future compatibility. External wiki and issue tracker can't be enabled by default as it requires additional settings. Disabled repo units will not be added to new repositories regardless if it is in the default list.
 - `PREFIX_ARCHIVE_FILES`: **true**: Prefix archive files by placing them in a directory named after the repository.
 - `DISABLE_MIRRORS`: **false**: Disable the creation of **new** mirrors. Pre-existing mirrors remain valid.
+- `DISABLE_MIGRATIONS`: **false**: Disable migrating feature.
 - `DEFAULT_BRANCH`: **master**: Default branch name of all repositories.
 - `ALLOW_ADOPTION_OF_UNADOPTED_REPOSITORIES`: **false**: Allow non-admin users to adopt unadopted repositories
 - `ALLOW_DELETION_OF_UNADOPTED_REPOSITORIES`: **false**: Allow non-admin users to delete unadopted repositories