Improvements to the admin page

A few things that could be added to the admin interface:

• some way for the users to contact the admins in order to request "data owner" privileges. This means one way for the admins to receive such requests!
• a place to provide the reverse domain to use as root part of datasets UUIDs (see meteoio#963)
• as described in issue #21 there should be a tab to define global environment variables for all users.
• Although this is currently a minor issue, it might grow over time: people might have multiple accounts, depending on the identity provider they use (and they might move from one to another one). This means it would be nice to be able to merge accounts (we could let the users do the dirty work of selecting which datasets need to be kept).
• some way to customize the appearance (already changing some colors would be nice). -> In the documentation (Sysadmin -> Deployment -> Other steps)