Add possibility to manage tag texts

[Vayel]

From instance, if two automatically collected documents have the tags math and mathematics, we probably want them to refer to the same tag in the database.

Possibility to rename a tag.

@ogrergo I need to talk with you boy! Should we manage tags like we do for USLs?

[Vayel]

Display similar tags, i.e. tags which share at least one usl