excerpt/full should apply to archives too

[aristath]

Fixes #454

Summary

Change the description of the theme-mod to make it clear this applies to all archives, and apply to archive.php.
Previously part of #416, was later removed.

Quality assurance

• I have thought about any security implications this code might add.
• I have checked that this code doesn't impact performance (greatly).
• I have tested this code to the best of my abilities
• I have checked that this code does not affect the accessibility negatively

[SResok]

Code review: LGTM!