Fix API for member edition

[almet]

Fix for #1259. To merge after #1260.

• Add a test showcasing the problem
• Find and implement a fix

[almet]

I can run the failing test with

.venv/bin/pytest -W ignore::DeprecationWarning ihatemoney/tests/api_test.py -k test_member_edition_keep_data -s

[almet]

From what I understand:

• When no data is provided, the form falls back to its default values, rather than using the already-present data
• There is an extra form.fill(member) method on the form, which job is to explicitely do this. It's used in the web.py views

[almet]

This is actually manyfold.

• The solution for the weight was to pass the obj to the Form,
• For the activation, it was trickier: WTForms considers that no value for a Boolean field means it should be set to False, so I've been looking at the raw data to see if its present before using the data.