add Create Study Chapter button

[JaimeKurzg]

Add ability to create a new chapter while in a lichess study you own or contribute to.

[JaimeKurzg]

I would be willing to work on this

[tom-anders]

Already exists:

Not in the app though.

@JaimeKurzg do you intend to implement the full blown study editor, like in the web? That's a pretty big feature for a first time contribution, you might want to start working on #1133 first

[Boubou78000]

Not in the app though.

Sorry, I thought it was an issue from lichess-org/lila

[JaimeKurzg]

@tom-anders I will probably do some of the easier issues before this one.