Use the `form-input` and `form-input-focus` mixins as the basis for all
input fields in the sidebar. This removes some duplicated SASS and
ensures that all input fields provide affordances look and behave a
little more consistently.

 - Move `:focus` and `@media (pointer: coarse)` adjustments for form
   inputs that were duplicated in several places into the `form-input` mixin
 - Change the `MarkdownEditor` and `SearchInput` components to re-use
   the `form-input` mixin for their input fields
 - Remove the unused `form-textarea` class

The end-user visible results of this are:

 - The search input in the top bar now looks consistent with other
   inputs when focused
 - The input field in the share dialogs now use a larger font size on
   mobile devices, which should avoid iOS zooming in when they are
   focused