feat(install): Replaces index dialog prompt with sidebar buttons

[castaway]

Fixes #1509

"Download" button how starts the index download, and greys out the button. While downloading, the "Cancel" button can be used to cancel the process. (This can take a moment or two as its only interrupting the download between files). Once either finished downloading, or cancelled, the dialog goes away.. and the usual "Synchronize/Stop synchronizing" appears under the folder list.

[gtandersen]

Excellent, this is a big improvement and especially if we can also make the index start downloading automatically when the page loads. That would allow the user to experience Runbox 7 optimally, which will be increasingly desirable with the growing number of up-to-date indices.

We then only need a "Cancel" button and if that is pressed it can be changed to "Delete" in order to allow the privacy concerned user to stop the download and/or delete the index. :)

Ideally there would also be a "resume" button, but that's another issue (https://github.com/runbox/runbox7/issues/1046).

[castaway]

We could.. should we? At least on mobile devices folks might not want a bunch of data download?

[gtandersen]

I think the app should be used to the fullest of its ability, and ideally/eventually download the index transparently in the background without disrupting the user. It's similar to IMAP in that regard, and even without the index will cache data locally anyway.

We could probably detect the device's data connection type and prompt the user if it's not on wifi.

[castaway]

Suggested location from @gtandersen