Moved several hard-coded UI strings to strings.xml

I noticed some strings were not localised when my phone was set to French, and then realised that these strings were actually hard-coded. So I moved them all to strings.xml, which also required some minor code changes to get the context in scope, in order to fetch the string in some cases.
Could we changes this to
` <string name="GroupUtil_joined_the_group">%s joined the group.</string>`
and
`description += String.format(context.getString(R.string.GroupUtil_joined_the_group), Util.join(members, ", "))`

There are (at least in German) grammar issues if we force the string to start  with the joined members.