[Terminology] Replace occurrences of "master" with "primary"

[perry-mitchell]

Replace occurrences of "master" with "primary" or something else more appropriate. Datasources/Credentials will be affected.

[wolffles]

Whats wrong with calling it master? Will the replacements be throughout all of buttercup-core?

[perry-mitchell]

@wolffles Yeah this was never intended to be a high priority change, but a potentially necessary one as the terminology (at least in tech) is in somewhat international contention at the moment. See the python master/slave terminology update.

We may not adopt this at all, so I changed the status to pending. Imho Master/Slave conveys no negative connotations when used in tech but there are for sure others that have different opinions and may be offended or put-off by their use in code/UIs. We use "Master Password" quite prominently in our UI, and although we don't use the term slave to my knowledge, the two go somewhat hand in hand.

This was just intended to avoid potential issues with terminology as there are perfectly fine alternatives. Let's see how we go..