txt-preview: improvments

[dfdan]

:heart: Thank you for your contribution!

Description

• Fixes horizontal scrolling in preview iframe
• Adds option to truncate .txt preview after a specified number of bytes

Fixes invenio-app-rdm#1864

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[dfdan]

Thanks @fenekku, good thinking on the possible multi-byte errors. I've added some tests but it looks like test_macros.py fails all tests anyway :( Tom has demonstrated it works however.

It would be good if we could merge this and @tmorrell's #179 into a 1.3.10 release (as well as push these changes forwards into 1.4) - so there is a painless upgrade option for existing installs.

[fenekku]

Looping in Karolina from CERN. Once this one and Tom's one are merged let do a release and backport to 1.3.10 like Dan mentioned.