Add ability to create json files from issues

[nvdaes]

The planned procedure to submit add-ons to the addon-datastore repo has security issues, as shown in:

nvaccess/addon-datastore#35

This intends to provide a way to add and validate add-on metadata by creating issues in addon-datastore, which maybe automerged if checks pass.

[feerrenrut]

Aren't most of these checks already done on the PR validation? To resolve the security issue, can you added the extra check to the PR validation. The extra check is:

If an add-on has been previously submitted, ensure the owner does not change.

Separately, the issue form/template would be convenient for submitters, and should be able to share validation code with the PR validation.

[nvdaes]

Most checks are already done. We can remove the check for publisher if none should change a sent file, even owners, throwing an error if status of file changed is "modified":

https://api.github.com/repos/nvaccess/addon-datastore/pulls/35/files

I'l try to do it in the yaml file. Also, I think that something should be done to prevent people to send add-ons compatible with API version not published yet. Even if they don't appear in the NV Access server.

[feerrenrut]

Github seems to have closed this when I tried to update the branch. I'm going to open a new PR and progress this.