Obey relevant max_length when importing strings from the XPI

[diox]

Fixes: mozilla/addons#15268

Linter validation isn't applied correctly for localized strings, and no validation is applied to the description field (which becomes the summary in AMO). This silently truncates the strings when importing manifest data to avoid inconsistent data and errors that are difficult for developers to recover from themselves.

Testing

• Uploading an extension with a description of more than 250 characters in the manifest
• Uploading an extension with a translation for the name in the manifest of more than 50 characters
• Uploading an extension with a translation for the description in the manifest of more than 250 characters
• The above, but with a (otherwise valid) default_locale in the manifest pointing to a locale that has no messages.json (or an incomplete one) in the xpi
• The above, but with an invalid default_locale in the manifest

For all these cases, if the string goes over the max for the field, the value should be silently truncated. The linter may rise proper errors for these cases in the future (See mozilla/addons-linter#4910 for instance)

[eviljeff]

It's unfortunate there isn't a more optimal way of implementing this - we're doing essentially the same retrieval of max length and string slicing in 3 different places.

[diox]

Yeah, the whole thing really needs a deeper refactor. Next time...