Script contributor tests

[stevegt]

We keep regressing in CONTRIBUTING.md. This time, it's starting to look like at least some contributors may not have been able to run drone tests since October 2018 (#6243). The last time we touched this was back in #4053, #4055, #4070, and #4071.

I think we need to:

• Script the drone testing that we're asking contributors to run, including fetching the right version of the drone cli tool, as well as the command line for running it.
• Change the instructions in https://github.com/go-gitea/gitea/blob/master/CONTRIBUTING.md to ask people to just run that script.

Both of these are handled by #6305.

[stevegt]

I've trimmed this issue; it originally included a suggestion that we run the script automatically at PR submission to ensure the script itself stays updated. If that gets done it should probably be handled as a separate discussion and/or project.

[stevegt]

I've removed the script from PR #6305 -- if anyone is looking for it, you can grab a copy at https://github.com/stevegt/gitea/blob/drone-cli-bash/scripts/test-local.sh.

The script only runs on Linux and should be replaced with a go version for portability; I'll work on that if nobody else gets to it first.

[stevegt]

Touching to make stale bot happy -- have been getting myself up to speed on go while working on another project; haven't forgotten about this.

[lunny]

Now we have used drone 1.x, so the CONTRIBUTING.md should also be updated again.