Feature: add some customization options

[blmage]

Some ideas:

• Disable the similarity matching (to save some performances),
• Disable the sorting on big solution lists,
• Disable the display of solutions in the forum comments (to save some storage / bandwidth / performances),
• Enable the inclusion of automatic tokens in the solutions (excluding or including the solutions with "fully automatic tokens"?),
• Specify the storage quota for challenges,
• Specify custom pagination options,
• Enable the logging of errors (currently reserved to the development builds), or even various debugging messages (with different levels : errors, warnings and notices),
• Specify options for the similarity matching (see Feature: Improve the calculation of similarity scores between answers and correct solutions #2)
• Enable viewing the solutions before submitting one's answer? (perhaps only the most similar?)
• Anything else?