Easier selection of user's own topics when making learning paths

[fbacall]

Summary of changes

• Adds a dropdown list of user's own topics in addition to search field.
• Moves some hard-coded text into translation files.

Motivation and context

Generally users want to add their own topics to a learning path.

Screenshots

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.