* config file store
Introduce an interface and concrete implementation for accessing the config.
This mostly maps 1:1 with the exiting usage in `App`, except for internalizing the watcher. A future change will likely eliminate `App.PersistConfig()` and make this implicit on `Set` or `Patch`
* experimental file test changes
* emoji: move file driver checks from api4 to app
It is no longer possible to app.UpdateConfig and provide an invalid configuration, making it hard to test this case. This check doesn't really belong in the api anyway, since it's a configuration validity check and not a permissions check. Either way, the check now occurs at the App level.
* api4: generate valid public link salts for test
* TestStartServerRateLimiterCriticalError: use mock store to test invalid config
* remove config_test.go
* remove needsSave, and have Load() save to the backing store as necessary
* restore README.md
* move ldap UserFilter check to model isValid checks
* remove databaseStore until ready
* remove unimplemented Patch
* simplify unlockOnce implementation
* revert forgetting to set s.Ldap
* config/file.go: rename ReadOnlyConfigurationError to ErrReadOnlyConfiguration
* config: export FileStore
* add TestFileStoreSave
* improved config/utils test coverage
* restore config/README.md copy
* tweaks
* file store: acquire a write lock on Save/Close to safely close watcher
* fix unmarshal_test.go
* Refactor context out of API packages
* Update function names per feedback
* Move webhook handlers to web and fix web tests
* Move more webhook tests out of api package
* Fix static handler
