clarify / unify calendar permissions evaluation

This was considered when reviewing MDL-67300.

Right now calendar has different methods deciding which type of events can be managed at every context. And it seems that they are used in a mixed way.

A proposal to unify uses a little more way done there, see this commit:

https://github.com/lameze/moodle/commit/44dad9ee5f0fd07f08fc593ef2fd6a5c18a3998d

And it was a good step, but this issue is a little bit more ambitious:

1) We should review all the methods within calendar that have responsibilities calculating permissions.

2) We should unify uses as much as possible, clearly deprecating the methods that we consider "old".

3) All the combinations should be well covered by automation tests.

4) The methods should be well documented (phpdoc is ok), about which are they correct uses and so on.