Add mention of mopidy-convert-config
docs: Fix typo and level of header
docs: Use 'config' more consistently
docs: Remove mixers' module docs
docs: Add section on extension config and disabling
docs: Add how to get good sound on raspis (fixes: #299)
docs: Review Raspi sound quality howto
docs: Add glossary (fixes #430)
docs: Add how to get good sound on raspis (fixes: #299)
docs: Review Raspi sound quality howto
docs: Fix typo
docs: Fix typo
js: Use 'npm start' to run 'grunt watch'
js: Update dev dependencies
config: Add keyring module with fetch and set.

New config submodule for fetching secrets from a secrets service over dbus. To
facilitate this we add a section and key to the stored attributes, allowing us
to fetch all mopidy values from the keyring and map them to the correct config
values.

A helper for setting values is also added. Due to differences in in the secrets
dbus API across versions of the APIs and various states of support in the
secrets service we try and open the default, login and finally session
keyrings for storage.

Locked keyrings will be dismissed for all operations.
config: Integrate keyring values with overrides.
config: Keyring debug logging cleanup
config: Review fixes.
Merge pull request #449 from adamcik/feature/keyring

Add keyring support via dbus secret service
core: Add last_modified to track model