Move webassets structures to constexpr

clang-tidy warned on some data structures that, if they throw, the
exceptions can't be caught.

Move these data structures to constexpr equivalents to save some memory.

Tested:

Loaded webui.  Worked as intended, and static files loaded properly.

Signed-off-by: Ed Tanous <ed@tanous.net>
Change-Id: I331ebfc2451f0cc0a82a1b70d325008c9c80401a