Fix search block widget

[IanDelMar]

Description

This PR

• uses a filter to add Bootstrap classes to the search form.
• fixes the button position styles.
• fixes the label styles.

Motivation and Context

#1647 tried to fix #1533. However, it added a lot of CSS, makes <label> permanently invisible and introduced unexpected styles.

Reduction of CSS file size

• theme-bootstrap4.css: from 570 KB to 368 KB.
• theme-bootstrap4.min.css: from 488 KB to 313 KB.
• theme.css: from 418 KB to 334 KB.
• theme.min.css: from 349 KB to 281 KB.

Bootstrap 4 example

Before this PR

After this PR

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer cs:check has passed locally.
• composer lint:php has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Related to #1647
Fixes #1533

Further comments

[IanDelMar]

@bacoords this PR needs a review.

[bacoords]

Great @IanDelMar - I'm going to pull in the latest BS version from #1821 and then test all the style fixes

[IanDelMar]

@bacoords I'll resolve the conflicts that will then occur.

[IanDelMar]

@bacoords I resolved the conflicts and updated the PHPStan baseline. I'll work on the "no value type specified in iterable" errors at some point in the future.

[bacoords]

This is great- I'd never even played with the new search widget. I thought only the 'legacy' search widget was available.