fixes lots of missing & inconsistent strings, as somehow some of the i18n files had ended up with the raw strings being inconsistent in terms of punctuation, causing problems as per https://github.com/vector-im/riot-web/issues/4045
* Serve translation files from the karma server
* Port UserSettingsStore to ES6 exports because the test runner
gets confused by ES6 importing a commonjs module
* Remove extra spaces in translations strings for MELS
* Fix 'his/her' back to be 'their'
* Change test to expect singular 'invitation' for a single person
(there may be multiple invitations, but IMO this should be
'rejected n invitations' and we can play with the wording later,
I don't think the singular is any worse than the plural).
* set language in the MELS tests (and wait for it to complete)
* Don't bother setting lang in other tests for now
