Make clear to the user that they need to give a *valid* commit/tag and have a documented license

[akien-mga]

@vnen and I are wasting our time reviewing new asset submissions which usually meet one of those two conditions:

1. User picks some license name on the assetlib edit (99% of the time MIT, i.e. default one), but they don't document any license in their repo.
2. User sets v1.0 as download commit, which doesn't exist in 99% of the cases because they don't know they have to push a tag to their repo.

It would be great if we could improve the UX of the submit page to make it clear for users that they need to include a license in their repo and to give a valid commit/tag string for download.

Some ideas:

• For (1), don't preselect the MIT license. Have nothing selected by default (and keep a valid selection mandatory of course). This at least ensures that people don't go with MIT unknowingly just because they didn't read. Edit: This is #138.
• For (1) and (2), improve hint to be more explicit about what they need to do on their end:
  • (1): Add something like "Your repository must contain the equivalent licensing information, usually with the full text of the chosen license in a LICENSE file."
  • (2): Add something like "If using a tag name, make sure that you actually created this tag on your repository."
• For (2), maybe precompute the download URL based on the user input and show it at the end of the hint with a clickable link. Ask user to verify that it's functional.
• For (2), test the download URL before allowing the submit. But it might be an issue to download stuff on our server each time we want to validate something - maybe the GitHub API offers a query to verify a download URL without actually downloading contents?

[vnen]

But it might be an issue to download stuff on our server each time we want to validate something

Maybe we can send a HEAD request to get the headers and see if it's a 404 without downloading the file.