Refactored all feature flags to use the new `useFeatureFlag` hook.

Also added a new functionality to this feature flag: ability to define
custom "default" value for a feature flag. This will allow us to enable
the new autocomplete component for everyone using Netlify builds
eventually (need to address some issues before doing so).

- Add a “hide decimal places” setting to visually hide the `.xx` from
currency values globally
- When hiding the fractional digits, slightly decrease character spacing
to allow more digits to show up

Ref: #327

New settings:
<img width="566" alt="Screenshot_2023-03-17 14 19 46"
src="https://user-images.githubusercontent.com/25517624/225986815-b884b93d-02f9-48b3-a73d-d27f90b678cf.png">


Before/after:
<img width="149" alt="Screenshot_2023-02-27 21 47 07"
src="https://user-images.githubusercontent.com/25517624/222916856-21ab4f03-56c6-4b24-8fc1-ac4b883138b7.png"><img
width="131" alt="Screenshot_2023-02-27 22 02 01"
src="https://user-images.githubusercontent.com/25517624/222916859-cf882ca3-6087-4994-818e-239c3374e412.png">

This implements a simple indicator showing the user if the currently
used version is outdated by utilising GitHub tags to track the latest
version.
Closes #463

---------

Co-authored-by: Jed Fox <git@jedfox.com>

* Allow the server to auto-configure the server URL for the client

* Extract server URL/version logic out to ensure consistent updates

* ()

* Be more explicit about when the server version is re-fetched

* Use a single layer of context provider

* Move the bootstrap route to /account/needs-bootstrap

* No more `isActual`

* Refactor to call subscribe-needs-bootstrap instead of fetch()

* Dedupe calls to subscribe-needs-bootstrap

* Don’t revalidate the server when we just validated it

* simplify

* Fix setServerURL

*  (prettier) adding trailing commas

* More commas

*   upgrade eslint-plugin-import and fix issues

* Change import ordering

* Make ButtonSetting more generic

* Add an “Experimental Features” section in the settings

* Fix text color

* Put experimental features behind another link

* Update Encryption.js

* Update FixSplits.js

* Remove unused variable

* That’s what I get for coding on my phone

Co-authored-by: Matiss Janis Aboltins <matiss@mja.lv>

*   enabling no-unused-vars eslint rule

* fix more lint issues

* Center the settings UI when the sidebar is collapsed

* Fix lint

* Split the Settings component into multiple files (#434)
* Remove need for isMobile in CSS: lean on media queries in styles.js and glamor

Co-authored-by: Matiss Janis Aboltins <matiss@mja.lv>
Co-authored-by: Jed Fox <git@jedfox.com>