Update issues and merge request templates

Description

• Remove redundant fields from issue templates
• Hide by default "Possible Fixes" field
• Fix labels assignment
• Provide a reference to correct issue linkage

Related Issue

Closes #25 (closed), #16 (closed)

BTW, the above is supposedly correct way of linking the issues addressed by a current merge request,

so they are automatically closed when this merge request is merged

How Has This Been Tested?

Checked the renders on gitlab

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.