Improve schemas, theming, validators and remote sheets harvester

[mjanez]

Here's a summary of the improvements made in this PR:

• Improved the package_item template for better UI presentation.
• Replaced status_show with package_list for CKAN API endpoint to enhance data retrieval.
• Updated styles for better user interface experience.
• Added schemingdcat_spatial_uri_validator to obtain spatial data from spatial URI.
• Improved schemingdcat_harvester schema for better data harvesting.
• Updated README for harvesters with more detailed instructions.
• Updated form placeholders with correct abbreviations for better user understanding.
• Refactored interfaces and improved docstrings for better code readability and maintainability.
• Added data-icons for provinces and province images based on autonomies for better data representation.
• Updated schemingdcat_spatial_uri_validator to handle missing spatial bbox, preventing potential errors.
• Added temporal_start and temporal_end fields to DATE_FIELDS for better date range representation.
• Improved spatial_uri form_snippet for better user interaction.

This PR aims to enhance both the user interface and the functionality of the application, making it more user-friendly and efficient.