Use consistent language for forum posts

First I also changed the string IDs in https://code.briarproject.org/NicoAlt/briar/commit/49227b0cef63205a2b90dfe5dbf08ace2c7ea875 but then decided to not do it in order to have less changes in total and probably less problems with translators.

I have also checked all code in briar-android/src/main/java/org/briarproject/briar/android/forum but did not find any code to change.