Use more specific button titles instead of general.submit

- "Post" for post submission (we already had the button title be "Posting"
when the post was being sent, so there was a weird inconsistency)
- "Register" for registration submission
- "Save changes" for usages in settings

src/components/registration/registration.vue

@@ -230,7 +230,7 @@
                 type="submit"
                 class="btn button-default"
               >
-                {{ $t('general.submit') }}
+                {{ $t('registration.register') }}
               </button>
             </div>
           </div>