Enhance Curation Flow: Source Existence Check, Directory Selection, GitHub Username Integration, and Source Info Fetching

[kouloumos]

Overview
The "Suggest a Source for Transcription" flow, introduced in PR #300, allows users to suggest sources for transcription, making them curators. Currently, the suggestion form includes a title and URL for the source,

and upon completion this type of PR is created.

This issue focuses on improving this flow by:

1. Source Existence Check: Implement a check to ensure the suggested source doesn’t already exist, utilizing the functionality already available on the transcription server.
2. Directory Selection: Currently, all new sources are automatically added to the "misc" directory. To improve this, we should allow curators to specify if their suggestion belongs in a particular directory. This can simplify organization, and for the implementation, we can potentially reuse the directory selection component previously used in the Editor interface.
3. Curator Identification: Allow curators to optionally provide their GitHub username for credit, potentially adding them as co-authors on the commit that curates the source. We need to explore how to implement the "Co-authored-by" pattern or consider GitHub login as a more streamlined solution.
4. Additional Information in the Modal: Include more details in the modal about the curation process, and provide a link to the About page for further context.
5. Source Information Fetching: Leverage the transcription server's data-fetching functionality to automatically gather and display information about the suggested source.
6. Multiple Source Suggestions: Allow curators to suggest multiple sources at once, streamlining the process for those who have more than one source to submit, so they don’t need to submit multiple times.

Implementation Notes
There is already an integration with the transcription server via #298 that can be further leveraged for (1) and (5).

Additional Feature Consideration
We can consider adding a simple weekly curation metric, displayed on the About page, to incentivize curator engagement. This could be dynamically updated based on their GitHub activity without requiring full curator profiles.

Next Steps

• Research and decide on the best method for adding curators as co-authors on commits.
• Implement the source existence check using the transcription server's existing functionality.
• Add an option for curators to select a directory for their suggested source.
• Enhance the curation modal with more detailed information.
• Integrate source information fetching from the transcription server.
• Add support for multiple source suggestions in one submission.
• Consider a lightweight curation metric display for added engagement.