Add Button to Download Public Gist

[auxves]

Short description of what this resolves:

This PR adds a button to the landing page to download public gist.

Changes proposed in this pull request:

• Add button to landing page
• Ask for gist id when clicked, and save it
• Enable downloadPublicGist

Fixes: #876 (comment)

Checklist:

• I have read the contribution guidelines.

[shanalikhan]

1. We need to run the Download Process also
2. We need to empty the current GIST, input box should prompted like old way to allow user to add GIST and hit enter to download. ( if we can use UI for this purpose only, your suggestion on this ? )

See this :
https://github.com/shanalikhan/code-settings-sync/blob/master/src/sync.ts#L979
https://github.com/shanalikhan/code-settings-sync/blob/master/src/sync.ts#L819

[auxves]

@shanalikhan

We need to empty the current GIST, input box should be prompted like the old way to allow the user to add GIST and hit enter to download.

This is already part of the PR. When the link is clicked, an input box is shown where the user can input the gist id. When they hit enter, it will save that gist id to the extension config and download.

if we can use UI for this purpose only, your suggestion on this?

I think it would be much easier to just use the input box as it's much quicker than informing the user how to do it in the Settings UI.