Introduce last_strip_mode option

Like the other similar options, this will be used to restore the state
of the radio button in this future dialog.
Introduce BrowserStripper dialog

This is a shell right now, but will be invoked when a user requested to
strip audio off of cards that they have selected in the browser.
Allow menu actions to suppress shortcut key

This will be used for the new menu item for removing audio, which will
not have a shortcut key.
Wire new remove audio menu into card browser

This is done by extending the already-existing hook and updating the
triggers to disable/enable it.
Validation and option saving for BrowserStripper

Adds checks to make sure fields are selected and saves back the
operation mode.
Separated addon.strip.sounds callables

This now distinguishes between an "AwesomeTTS technique" and a "Anki
technique". The former will be usable by most of our code, and will use
a precompiled re object and eventually have more functionality, whereas
the latter will be used by code that needs to emulate something that
Anki does.
Add stripping support

Implements the accept() method for the Stripper dialog.
_accept_next_output broken if file already present

There was not a return code path for this case.

(cherry picked from commit d9c294ede0e88f15c296cb0dc6cf9d8794292532)
Disable user interface while stripping

This makes it so if the user is doing a big batch, it doesn't appear
that they can interact with the UI.
Update card browser documentation

Because the menus have changed, need to update copy and imagery on this
page.
Change "Okay" to "Remove Now" on Stripper dialog

This helps clarify what the button does and makes it consistent with
others.
Improve messaging after stripping notes

Adds information about purging disused media files, and makes some
messages conditional.
Add accelerators for stripper radio buttons

This will make it quicker for heavy keyboard users to toggle the radio
buttons at the bottom of this dialog window.
Documentation for mass removal feature

Adds new Removal page to website and adds links from some of the other
pages.
Merge branch 'add/sound-stripper-47' into develop

Adds functionality for the user to remove audio references via the card
browser. Closes #47.