Ember Boostrap adds `.d-block` class to validation feedback in order to
make it visible for input-group in which the `.invalid-feedback` is not
a silbing of `.form-control`.
This fixes the invisble validation feedback on /create/options (poll type:
answer a question) and /create/options-datetime (poll type: find a date).
* Adds a loading spinner to form buttons as long as submission is pending.
* Does some refactoring of form navigation buttons.
* Updates expected bundle size.
- Drops floatthead and additional scrollbar
- Makes header and first column sticky
- Refactors code for readability
Sticky header is only working in Firefox. Chrome and Edge does not support `position: sticky` for `<thead>`. Haven't tested Safari.
