Design changes to search functionality

[toolness]

We want the search functionality to basically allow users to change the following example sentence:

I need help with [measuring] [open data] in the [education field] in [Mexico].

1. The first field, [measuring], is the category of skill. This is identified by the currently-unused topic key in questions.yaml.

Batu and Andrew will be changing the topics so that they are the same across all questionnaires (e.g. every questionnaire will have a "measuring" category). However, according to app/__init__.py, changing the topic of a question will also change its id in the database; that will massively break our existing data unless we create a migration, or store this refactored category in a separate field, or something else.

2. The second field, [open data], is the questionnaire area.
3. The third field, [education field], is the "fields of work" field that already exists in our search interface.
4. The fourth field, [Mexico], is the "country" field that already exists in our search interface.

It should be noted that our current search interface also has a "languages" field that isn't reflected here.

The search results should ideally be organized into practitioners/explainers/connectors/peers tabs the same way as the "Match Me" results, and they should be ordered first by number of skills.

Ideally there should also be a full-text search field that searches across all the text fields in the user model (organization, name, projects, etc).

[toolness]

Added full-text search in a8e39a2bca7cd30d4793966f02c8156ca2e0fff3.

Made the language and field of work fields single-select instead of multi-select in 7876cdb598ed77cff4fbe9055024d57433591aea.