refactor: align submit buttons consistently

@alina-beck said we have most buttons left-aligned, so I went with that:
https://github.com/Human-Connection/Human-Connection/pull/1711#discussion_r329505767

Also this uses icon `envelope` for emails. This makes sense, because we
could use icon `at` for slugs.

webapp/pages/settings/index.vue

@@ -32,7 +32,6 @@
         />
         <template slot="footer">
           <ds-button
-            style="float: right;"
             icon="check"
             :disabled="errors"
             type="submit"

webapp/pages/settings/my-email-address/enter-nonce.vue

@@ -5,7 +5,7 @@
         <ds-input
           id="email"
           model="email"
-          icon="at"
+          icon="envelope"
           disabled
           :label="$t('settings.email.labelNewEmail')"
         />

webapp/pages/settings/my-email-address/index.vue

@@ -8,7 +8,7 @@
   <ds-form v-else v-model="form" :schema="formSchema" @submit="submit">
     <template slot-scope="{ errors }">
       <ds-card :header="$t('settings.email.name')">
-        <ds-input id="email" model="email" icon="at" :label="$t('settings.email.labelEmail')" />
+        <ds-input id="email" model="email" icon="envelope" :label="$t('settings.email.labelEmail')" />
 
         <template slot="footer">
           <ds-space class="backendErrors" v-if="backendErrors">
@@ -106,9 +106,3 @@ export default {
   },
 }
 </script>
-
-<style lang="scss" scoped>
-.submit-button {
-  float: right;
-}
-</style>