Make prompts customizable and extensible

[dkastl]

Currently the two prompts for the assistant buttons are hard-coded.
In order to make the assistant plugin customizable and extensible a similar approach as https://github.com/gtt-project/redmine_text_blocks could be chosen.

1. Select assistant prompts from a dropdown list

2. Select assistant prompts from a button list
3. Create and manage a list of assistant prompts globally or on a project level

GTT Assistant can take a similar approach as the Text Blocks plugin, it may even become an alternative or replacement.

The following information may be filled in for an assistant prompt:

• name: the name that will also be shown in the dropdown (or as button)
• type: to specify where the prompt should be placed, for example "button" or "dropdown"
• prompt (previously text can be a set of "messages", where messages can be assigned a role (either "system", "user", or "assistant"). (Needs to be researched more)
• context: allows to specify what context should be taken into account, for example "issue description", previous note, all notes, etc.
• issue statuses: to be inline with the text blocks plugin
• locale: not sure if we need to be able to limit the prompt to a language

We could even make the use of ChatGPT optional (by adding a checkbox) and then use the prompt in the same way as text blocks are used currently to just provide a template.