Resolve "TNW Group Matching priorities"

Added the ability to prioritise certain students in the algorithm.

When prioritised students are placed in a couple with a non-prioritised (or less prioritised) student, the higher priority applies to the whole couple.

Added a tnw_weight_bonus field to User to control this bonus.

The bonus works by increasing the weight of each of the priorities of this student. For example, the algorithm would treat giving a student with priority 2 their second choice the same as giving an unprioritised student their 4th choice. Setting this to a high value (e.g. 1000) would effectively mean that those students will definitely be assigned a project, likely one of their first choices (if the number of prioritised students is small).

To have multiple priority groups, the highest priority group should have the highest bonus value. For example, with three priority groups, the highest priority group should have the bonus set to 2000, the next one to 1000, and the last one should not have it set.

Related to #582 (closed)