Update legacy themes

Problem/Motivation

There are currently 3 themes that originate from Opigno (see lms_theme()):

• lms_lp_step_activity
• lms_lp_step_module
• lms_lp_step_module_activity

All 3 are used for a single element: course navigation block that usually is placed in a sidebar. We can simplify that as much as possible, possibly use a single theme and template for the entire thing and rename so we don't have "lp" anywhere.

Details to be refined by a frontend engineer.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes