[token-server] Bump icon upload byte ceiling

Summary:
Does not seem to be happening often, as Cashtab resizes icons to 512x512 before upload, but seems like some users are hitting this.

Bump 1mb size limit to 2mb.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17273