Enforce maximum height of signwell fields

Enforce maximum height of signwell fields

Signwell have implemented a size limitation that no fields can have a
height bigger than 34px. Unfortunately, their own GUI editor allows the
creation of fields that are larger than this, and once one has done that
they can no longer be loaded for editing or sent through the API (it
appears the limitation is only in the public API and not their internal
ones).

So to make signing work at all, when editing a contract enforce the
limit to 34px (with a warning). Things might not look very nice anymore
but they should at least work.

Support case has been opened with Signwell to see if this was actually
intentional (the max size in the UI editor appears to be 74), and if so
if they are planning to align the UI with the API. This commit goes in
pending a possible change on their side and we can revert if if we end
up not needing it permanently.