VideoPress: expose allowed mime types of the site to the client

[retrofox]

Probably, we can get the allowed mime types by using get_allowed_mime_types() core function and expose it in the Initial state global var.

[lhkowalski]

Today we have a fixed array of allowed video formats here: https://github.com/Automattic/jetpack/blob/trunk/projects/packages/videopress/src/client/state/constants.js#L31

We need to use the get_allowed_mime_types() to get this list of extensions from a better (and more maintanable) source of true.

[lhkowalski]

get_allowed_mime_types() returns a list of extensions and associated mime types (ref).

To get a list of possible video extensions, what we need to do is fetch all of them and filter for the video/* mimes.