Forum creation bug fix

[Mat-san]

The associations in the forum entity class were incompatible with their intended use. According to the annotations, each user would only be subscribed to one thread, however, it's clearly many-to-many as each user can be subscibed to multiple threads and each threads can have multiple subscribed users. Also, fix the entityToDto forum conversion to also include the author.

[christopher-boustros]

Finished with commit 436ccb99125045e26266a427cfb80ed6c01a2c6c