[EXTERNAL USER][ISSUE] - Allow External Share user to create issue

Description

Currently external users are unable to create issues and are limited to editing existing issues.

Suggested improvement

Enable external users to create issues using external share interface

Decision on implementation
Creation will be possible from board share. For starters, no additional share will be created for every new issue, created issue will be accessed through board share (configuring the board so that new issues are on it is responsibility of the client). Only basic data can be set during creation, but more can be added afterwards during edition (provided that edition is turned on in configuration).

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