Updated username validation rules

[felixoi]

I discussed username validation a bit with Snowie after we got a user with a dot in the name which introduced some trouble.

New username rules would be:

• The username can contain the following characters: a-zA-z0-9-_
• The username cannot start or end with -
• The username cannot contain two special characters in a row (e.g. __test__)

Allowed examples:

• _test_
• bettertest_
• this-is-a-test
• iamprettynormal
• I_am_special

Not allowed anymore:

• I.am.a.test

Before we also allowed usernames to contain . which will be disabled with this PR.
However this PR now allows usernames to end with _. This is maybe controversial but since ewoutvs_ has this in his name as staff member, we should allow it.

[coveralls]

Coverage decreased (-0.01%) to 96.453% when pulling 5dd0ba1 on fix/username-regex into 0875cf8 on master.

[gabizou]

While the worry of letting people potentially confuse users with names like _gabizou or _gabizou_, I feel that staff as a whole could moderate those users since the user who tries to register with that name would more than likely be doing so facetiously.