Allow astropy.table.join to be used with TimeSeries objects

Currently we can't use astropy.table.join with TimeSeries objects because it only works with non-mixin columns. It would be nice to investigate whether we can get it to work since combining multiple time series in this way would be useful (see #8540 (comment) for an initial discussion)

I have a deja vu here, and will try to dig up whether it was discussed in another issue or I started to work on it somewhere...

This has been done!