TB 128 support

[anttiisoaho]

Seems not to work properly with TB 128:

• CSS is added properly into new emails
• CSS is not added into replies/forwards

[GrantEdwards]

TB 128 hasn't gone "stable" on my OS yet, so I only have TB 115 available.

[GrantEdwards]

I've upgraded to TB 128.3.0esr (64-bit) on Gentoo Linux, and I see the custom CSS in new email, replies, and forwards.

[GrantEdwards]

I repeated the test with TB 128.3.0 on a different Linux machine (slightly different set of extensions installed), and custom CSS shows up in new, reply, and forward emails.

[anttiisoaho]

Got it - seems that there is now some conflict with ReplyWithHeader extension - when both CustomCSS and ReplyWithHeader are enabled, this conflict is faced when replying or forwarding. Interestingly both these extensions worked perfectly together in TB version 115.