Add contact users facility

**Is your feature request related to a problem? If so, please describe.**
Yes, we always have to find ways to contact project managers, translators and depend on third-party services to get touch with them however crowdin, transifex and other popular platforms offer this feature already.

**Describe the solution you'd like**
I'm not sure if that make this platform unwanted heavy and complexity for self-hosted instances.
If implement then it's necessary to have contacts list, inbox, sent, archive, trash, label facilities in that component

**Describe alternatives you've considered**
This's easy, let to make first contact from weblate. See the sample on https://launchpad.net/~flexiondotorg (random user)
It's necessary to login to account for see that contact option and form


I guess that is convenient, once made the first contact then able to continue with weblate account registered email
(Also add option to toggle for send a copy of it to own mail address for necessary users)