Rename account to label where appropriate

Summary:
This change only updates strings and adds RPC aliases, but should simplify the
implementation of address labels in
https://github.com/bitcoin/bitcoin/pull/7729, by getting renaming out of the
way and letting it focus on semantics.

The difference between accounts and labels is that labels apply only to
addresses, while accounts apply to both addresses and transactions
(transactions have "from" and "to" accounts). The code associating accounts
with transactions is clumsy and unreliable so we would like get rid of it.

Backport Core PR11536
Depends on D1980 and D2001

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox, Fabien, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, Fabien, deadalnix

Subscribers: deadalnix, Fabien, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1986