[FAU-446] Support for Multi-Word Free Text Search

[zhyian]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
https://inpsyde.atlassian.net/browse/FAU-446

What is the new behavior (if this is a feature change)?
The search algorithm has been updated to handle multi-word queries. The search results now include degree programs where all searched terms appear in any order within the relevant fields.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
No

Other information:

[o-samaras]

Code-wise LGTM. I 'm afraid this will have a performance hit when longer phrases are used but shouldn't be too obvious or cause issues on the search functionality. Thank you for working on this!

[shvlv]

I agree with Orestis that MySQL is not the best engine for full-text search, but we replicate WordPress's core functionality. Thanks for implementing!