Allow to log out

[Sobak]

It's probably doable by just going to /login/ but it should be possible to change the connection without touching the configuration.

[Albert221]

0972b9a adds ability to connect to a different database by going to /login and to return to configuration file connection by going to /logout.

It still needs some polish though like adding a hyperlink to the layout.

Also, there is need to implement the functionality of allow_overrides option (@Sobak, do you have time?).

[Albert221]

allow_overrides does not provide any functionality right now, but except that everything works as should.

As of d1cb8aa the primary functionality is present.