Enable edit button by default

There are two intended audiences for this change:
1. Sites that already have the module installed; these won't want any unexpected changes to their configuration, so wouldn't want every field suddenly enabled.
2. Sites that are adding the module for the first time; IMHO these would want the feature enabled by default.

Both audiences could be handled as follows:
1. Set the default to be "on", which is what the merge request currently does.
2. Add an update script that sets the value for existing sites to "off".

mrshowerman → changed the visibility of the branch 8.x-1.x to hidden.

I tried to create a new issue fork based on 3.x, but failed. Thus, I'm uploading a patch.
Addressed most of the feedback on the MR and also #10.

Why is the new setting not declared as a boolean?

@mrshowerman I tested your patch. It is doing what expected. There is just one tiny issue I have found. The setting page `/admin/config/media/media-library-edit` is only accessible from the widget setting of each field. But should be also visible in the menu system.

mrshowerman → changed the visibility of the branch 3.0.x to hidden.

mrshowerman → changed the visibility of the branch 3294593-enable-edit-button to hidden.

Created a new branch based on 3.0.x and merged in the existing work. (#9)
I noticed that if we have the option to specify a form mode in the widget, we should also have an appropriate global setting that is used when the default setting is used.
Specified a menu link as per #14 – the position in the menu tree is to be discussed, of course. I put it below "Media library settings", but we could also use one level higher, below "Media".
Added settings schema (#13) and update script (#10).