Revert minimum 50 kg limit

[rmsthebest]

Short people can be healthy at lower weight.

If you want to enforce a weight maybe use BMI instead of one limit.

[gwosd]

That is a good point that we should address.

I like the idea of restricting BMI rather than absolute weight. @braniii What do you think?
However, I would not expect this to change any time soon as we are both very busy at the moment. In the meantime, it is currently only forbidden to set (and not to have) a target weight <50kg. So a workaround would be to install an older version, set it there and then update again.

[braniii]

If downgrading please remember to backup the measurements! @gwosd I like the idea about limiting the BMI and if no size is set using 50kg as fallback value.