Allow registering users where the username contains dots (.)
## What does this MR do?
- Allow registering users whose usernames contains dots `.`
- This can currently be done by registering with a username containing no dots, and then editing the username to have dots in the user's profile settings.
## Does this MR meet the acceptance criteria?
- [#24276/!7500] Unable to register names with dot
- [x] Implementation
- [x] Tests
- [x] Added
- [x] [Passing](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7500/builds)
- [x] Meta
- [x] CHANGELOG entry created
- [x] Documentation created/updated
- [x] API support added
- [x] Branch has no merge conflicts with `master`
- [x] Squashed related commits together
- [x] Review
- [x] Endboss
- [x] Use `Gitlab::Regex::NAMESPACE_REGEX_STR` instead of a hardcoded pattern
- [x] Define `NAMESPACE_REGEX_STR` in terms of `NAMESPACE_REGEX_STR_JS`
- [ ] Wait for merge
## What are the relevant issue numbers?
- Closes#24276
See merge request !7500
75c8faf7ba