fix: add milestone & merge checks to enum_checks_type

[Rechi]

I tried to enable Pull request milestone check for one of my repos at https://zappr.opensource.zalan.do.

It always fails with

Error during check processing
Check pullrequestmilestone already exists for repository [...].

Tried local with a commit before those checks to generate the old schema, same error after updating. Then I added the migration files and it worked.

If possible 6de017b should be also ported to the production version.

[fokusferit]

Thanks @Rechi for contributing. I just wanted to inform you that we are currently, focused on #560 and #556 once they are merged and running fine, we will be able to merge other PR's

[fokusferit]

@Rechi I should have done a closer look to this 😄 this exact issue we faced the last weeks. I fixed that issue in the master (added like you two migration scripts).

I'm therefore closing this and just check out the last master. If you think there is still sth missing, feel free to create an issue or open a PR.