replace icon defs with plugin specific names

[nathonius]

The git-pull-request icon is colliding with a built in icon for the same thing. I ran into this using the Obsidian Git plugin, and actually made a PR there before realizing it was this plugin causing the issue.

This PR just prepends jira-issue- to all the icon defs and usages to ensure there are no collisions. Replacing custom icons with the built-in ones would probably be better, but that was a larger change than I wanted to make without discussion first. You can see all the built-in icons using getIcons().