Generalize workaround for taskw character encoding

[scross01]

The Logseq and Todoist plugins both implement a workaround for characters " [ and ] getting URL encoded as &dquote; &open; and &close; in the taskwarrior task description and content by replacing the over-encoded characters with alternate ' 〈 and 〉 to avoid the encoding logic.

This issue is originally discussed here in the Logseq PR, and again here in the Todoist PR.

The root cause of the issue in the taskw python package ralphbean/taskw#172

While this issue remains unresolved in taskw, or until replacement package is used #1078, it's proposed to move the character replacement method to the base Issue class and apply it to the relevant fields in every service.

[ryneeverett]

I probably wont implement this as I'm more motivated towards solving #1078, but I'm open to merging it.

If this does get implemented, note that we could either add main/general configurations for replacement characters in config.schema.MainSectionConfig or add them as common service options in config.schema.ServiceConfig. Either way, the only impact on services themselves should be calling unescape_content on the relevant fields.