Add checklist item to PR template for addition of static assets

[DrGFreeman]

Description

Add a checklist item to the PR template to ensure that if static assets are added, they are included in the AppArmor profile, as well as in the securedrop-app-code debian package.

User Research Evidence

In #5593, icon files were added in a new folder at static/icons/ however, the new folder and its content were not added to the .rsync_filter file, resulting in the icons being from the Debian package. New issue and PR (#5625/#5628) had to be raised to add the folder and icons to the .rsync_filter file.

Also, the initial submission of #5593 did not add the icons to the AppArmor profile. They were added as part of the PR review process.

User Stories

As a external contributor, I want the PR template checklist to act as a reference so I can ensure my PR is complete, including changes I need to include that I may not be aware of.

As a PR reviewer, I want the checklist to be as complete as possible to reduce the number of details I need to remember to check when reviewing a PR. I also want to avoid follow-up PRs to fix details missed in the first review.