Restrict possible characters for hood names

[m-rey]

As hood names are used in URLs and email addresses, among other things, we have to make sure that the chosen hood name is compatible.

[maikeMai]

limitations we encounter/reasons to restrict possible characters:

• there are certain restrictions for the localpart of email addresses.
• sanitizing user input is important to limit attack surface; this is a user input.
• hood names are used in URLs.

to be researched: symbols which are allowed in email localparts, e.g. -._+ (+ is already a special case and gets interpreted, should be excluded)

Apart from that, we should limit possible hood names, because if it's possible to receive email for root@kibicara.net as an admin, we have a security problem:

https://webmasters.stackexchange.com/questions/104811/is-there-any-list-of-email-addresses-reserved-because-of-security-concerns-for-a#105020

[maikeMai]

Please revert c3d6f8bfab64ad0f6504aabd7f7a589385379464 ("[email] Prefix for email recipient address") as soon as this is implemented - then the prefix won't be needed anymore.