Unify button styles and use min-width

This seemed to be the same across multiple components anyway.

Switched to min-width to allow for buttons with longer text, e.g.
from other languages.

src/App.scss

@@ -63,6 +63,8 @@ button{
   box-shadow: 0px 0px 2px black;
   font-size: 14px;
   font-family: sans-serif;
+  min-width: 10em;
+  min-height: 2em;
 
   &:hover {
     box-shadow: 0px 0px 4px rgba(255, 255, 255, 0.3);

src/components/login_form/login_form.vue

@@ -34,11 +34,6 @@
 @import '../../_variables.scss';
 
 .login-form {
-  .btn {
-    min-height: 28px;
-    width: 10em;
-  }
-
   .error {
     text-align: center;
   }

src/components/post_status_form/post_status_form.vue

@@ -124,10 +124,6 @@
     padding: 0.5em;
     height: 32px;
 
-    button {
-      width: 10em;
-    }
-
     p {
       margin: 0.35em;
       padding: 0.35em;

src/components/settings/settings.vue

@@ -83,8 +83,6 @@
 
   .btn {
     margin-top: 1em;
-    min-height: 28px;
-    width: 10em;
   }
 }
 .setting-list {