The extension manager can now be given a list of extensions that must

always be enabled. These act like enabled-by-default extensions, except

that they cannot be disabled through disable_extension() or the UI,

and will be considered enabled every time the extension manager loads.
This and the enabled-by-default extensions can both be set in

settings, or can now be set as attributes on the extension manager,

giving implementations a central, non-settings-based place to customize

this.
Any always-enabled extension will still show up in the extension

manager, but it won't have a button for disabling the extension.

Similarly, the API will return an error when attempting to disable it,

if invoked through other means.

Unit tests passed.
Marked some extensions as always-enabled. Verified that they were

enabled on load/refresh, even when I forced a disable through other

means.
Verified that they could not be disabled via the UI. No button

appeared and API requests attempting to disable were met with an

error response.