Better support for ad-hoc workouts

[rolandgeider]

(prio low, nice to have)

Since not everybody plans their workouts in advance, we should accomodate this as far as we can:

• add some kind of custom mode to the gym mode where the user just logs whatever they are doing. The backend already supports adding logs that are not tied to a specific day or slot entry object, etc. but still would need to create a routine for this, though.
• make sure the log diary works properly with this (I think currently we iterate over the plan and show the logs for those exercises?). Depending on the randomness of the exercises, the charts are probably not going to be very helpful, but we could show the exercises and logs per selected date, similarly to what would be logged to a notebook.

[fredrmb]

We're a group of friends who'd find this functionality very useful! Especially good if the user interface makes it easy to search for different exercises and see exercise information with images/videos, without having to exit log entry menu. This way, the app would assist us (beginners) in choosing/experimenting with different exercises. Ideally, we'll log what we did, browse various options for the next entry, select something to log, and repeat until end of session.

Edit: just back from the gym, one key feature would be showing the history of previous weights+reps for any particular exercise, because we don't always remember...

Edit 2: we have finally created something of a routine, but it's not necessarily 100% fixed. We choose and pick a bit depending e.g. on equipment availability. So it would be nice if exercises could be added/removed on the fly when logging any particular workout, even if following a routine.