Support transaction signing without metadata

[prybalko]

we should remove a number of web2-style restrictions (presumably intended as safeguards, but ultimately centralising and authoritarian-friendly):

• no requirement for metadata to exist prior to signing a transaction - it should give a sufficiently obvious indication that signer cannot guarantee the implications of signing, but it must be up to the user. i'm ok for this to be opt-in, enabled only by a default-off global setting.

[camfairchild]

This is akin to blind-signing. I'm not sure this should be supported because I could see a user always using blind-signing, and not truly understanding the implications.

[prybalko]

The idea is to give users tools, not to restrict their options.
There will be a large pop-up window describing the potential hazard of blind signing, but it won't forbid the user from doing so. There could be multiple situations where blind signing might be okay. For more details: #904