Extend data model: multiple instances of same vocabulary

Configuration data

The current data model assumes that each vocabulary can be linked to an user once (just like every vocabulary can be assigned to a node just once).

There is a requirement to extend that, allowing to associate (terms of) certain vocabularies to an user multiple times.

Example:
- there is "country" vocabulary defined in taxonomy, vid = 1
- admin sets up a user<->vocabulary relation called "Where I have been". It is based on vid = 1 and has its own unique relation id, rid = 1.
- now admin sets up another relation "Where I want to go", based on the same vocabulary as previous relation, vid = 1, but relation has new id, rid = 2

To store the neccessary fields, configuration settings table "user_tags_vocabulary" should contain 2 additional fields, rid and label_profile.

Taking the example above, the "user_tags_vocabulary" data looks as follows:

rid     vid	
----------
1       1   	
2       1   	

Optional: we also need to store related labels, one for profile editing / view page, another for overview page ("people who have associated with term x")

for that 2 additional fields needs to be defined

rid	vid	label_profile	label_overview
-----------------------------------------------------------------------------------
1     1     "Where I have been"      "Users who have been in %a"
2     1     "Where I want to go"     "Users who want to go %a"

If the userwith proper permission visits his profile edit page, he can see following fields:

Where I have been
[                                 ]

Where I want to go
[                                 ]

User term data

To support storing term data for multiple instances of same vocabulary, new column needs to be added to the "term_user" table, rid.

If user (uid = 10) fills, say "Where I have been" field with "United Kingdom" (tid = 100) and "Where I want to go" with "Brazil" (tid = 200), the data in table term_user looks as follows:

vid     tid     uid     rid
----------------------------
1     100     10     1
1     200     10     2

Related API and UI changes will follow.