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/post_status_form/post_status_form.vue

@@ -272,7 +272,7 @@
           disabled
           class="btn button-default"
         >
-          {{ $t('general.submit') }}
+          {{ $t('post_status.post') }}
         </button>
         <!-- touchstart is used to keep the OSK at the same position after a message send -->
         <button
@@ -282,7 +282,7 @@
           @touchstart.stop.prevent="postStatus($event, newStatus)"
           @click.stop.prevent="postStatus($event, newStatus)"
         >
-          {{ $t('general.submit') }}
+          {{ $t('post_status.post') }}
         </button>
       </div>
       <div