Add issue templates

[steff456]

This PR is part of https://github.com/conda-incubator/conda-store-ui/issues/254

This PR,

• [x] Adds an issue template for bug reports
• [x] Adds an issue template for feature requests
• [x] Adds an issue template for general issues
• [x] Adds a pull request template

[kcpevey]

Blocked until https://github.com/conda-incubator/conda-store/pull/521 is merged and the changes are made here as well.

[trallard]

This is now unblocked

[steff456]

This PR needs an approval for merging! I already copied all the changes from the conda-store PR