Avoid visual glitch when terms appear for IM
This avoids a visual glitch where the Integration Manager portal would briefly
appear, but then be replaced by a smaller Terms dialog when there's something to
agree to.
To resolve this minimal code churn, this cheats a bit and customises the size of
the terms dialog to match the IM portal modal when terms are shown for IM
purposes.
Fixes https://github.com/vector-im/riot-web/issues/10386
