key_without_language() reimplemented with regexp

[atilantera]

Description

What?

This pull request contains several related modifications to A+ RST Tools issue #134.
#134

1. In lib/toc_languages.py, functions key_without_language() and join_keys()
   were refactored into methods in the class IndexJoiner. This is more consistent,
   as there are other "join"-named methods in IndexJoiner.

2. Method key_without_language() in lib/toc_languages.py had a bug: the
   function could only strip language ids of form "id_" and "id-", not "_id" or
   "id". The bug was discovered with course the CS-A1141 Data Structures and
   Algorithms Y. The function is now reimplemented with a more flexible regular
   expression.

3. Method join_keys in lib/toc_languages.py now prints a warning message
   into log. The message describes the location of mismatching keys in the
   documentation tree, and mismatching keys with their language identifiers.
   Example:

   Mismatching keys at modules.1.2.3:
   Language: en Key: sorting_en_something
   Language: fi Key: sorting_fi_different

This should help the course material author locate the error. Moreover,
as the method now logs an error message instead of raising a SphinxError,
the compilation process can show log messages of all mismatching keys at
one compilation run, thus increasing productivity.

4. Unit test file tests/test_toc_languages.py tests the aforementioned
   modified functions. The unit tests allow the A-plus RST tools developer to:
   (i) easily test the functions of the software without needing to run a
   Sphinx compilation of a test material, which can take minutes;
   (ii) document how the functions should respond to certain inputs.

Why?

A+ RST Tools could not produce a helpful error message when there was
something inconsistent with keys on multilingual courses.

How?

Described above.

Fixes #134

Testing

Python unit tests are introduced to A+ RST Tools in this pull request.
There is the new directory "tests".

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Selenium tests.
• Other test. (Add a description below)
• Manual testing.

[ADD A DESCRIPTION ABOUT WHAT YOU TESTED MANUALLY]

Did you test the changes in

• Chrome
• Firefox
• This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Translation

• Did you modify or add new strings in the user interface?
  Original error message:

  Corresponding RST file names must match in multilingual courses:
  fi_something
  en_different
  

  Modified error message:

  Mismatching keys at modules.1.2.3:
  Language: en Key: sorting_en_something
  Language: fi Key: sorting_fi_different
  

Programming style

• Did you follow our style guides?

Have you updated the README or other relevant documentation?

• documents inside the doc directory.
• README.md.
• Aplus Manual.
• Other documentation: tests/README.md

Is it Done?

• Reviewer has finished the code review
• After the review, the developer has made changes accordingly
• Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!