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/settings_modal/tabs/profile_tab.vue

@@ -153,7 +153,7 @@
         class="btn button-default"
         @click="updateProfile"
       >
-        {{ $t('general.submit') }}
+        {{ $t('settings.save') }}
       </button>
     </div>
     <div class="setting-item">
@@ -227,7 +227,7 @@
         class="btn button-default"
         @click="submitBanner(banner)"
       >
-        {{ $t('general.submit') }}
+        {{ $t('settings.save') }}
       </button>
     </div>
     <div class="setting-item">
@@ -266,7 +266,7 @@
         class="btn button-default"
         @click="submitBackground(background)"
       >
-        {{ $t('general.submit') }}
+        {{ $t('settings.save') }}
       </button>
     </div>
   </div>