feature: Add ability for the user to provide a download link

[sammcj]

Problem: Too many apps that require "Update in "

Potential improvement: Provide a way for users to add a download URL / download page to applications.

Example:

1. Latest can detect that AnythingLLM is out of date
2. The user clicks update
3. ...but can't update it so it offers to update in AnythingLLM by opening the app
4. ...however - AnythingLLM doesn't actually have a self-update capability
5. so the user has to browse to their website
6. click on the download page
7. click download
8. install/replace the app
9. re-launch it
10. refresh latest

= too many steps for one application, and there's a number of apps like this.

How I'd suggest this could be improved with Latest:

Let's use AnythingLLM as an example.

0. The user adds the download URL to latest (e.g. right click->add download link for application).
1. Latest detects AnythingLLM needs an update (as it currently does).
2. The user clicks update
3. Latest uses the download URL to either:
   a. (If it's a link to a dmg/pkg):
   1. Download the installer (), runs it and refreshes when complete/ejected.
   b. (If it's just a link page):
   1. Open the users browser to the download page, and leaves a dialogue open in Latest to offer to check again once installed.
   2. The user downloads/updates the app.
   3. Clicks refresh/relaunches Latest.

Could become something like: