Rename "LMS" related classes

[porduna]

In the beginning, we were talking about LMSs all the time. However, now that we support PLEs and CMSs, we should change the name of that concept.

Proposals:

• "Institution" (in Graasp, one "LMS" instance actually refers to a school, for instance. It may happen that there is more than one LMS in a single institution, but I'd rather say that it's still fine)
• "LT" (Learning Tool; which could be LMS, CMS, or PLE, even if in Graasp it does not refer to Graasp itself but to each school running on Graasp)

[ilequerica]

LT is ok for me. I rather prefer it to "Institution".

[accaminero]

Agree