explore menu icons

[floriangantner]

additional icon for explore menus. The menu entry can be some internal or external link with an additional icon. The icon can be configured for each explore_section

References

• Fixes https://github.com/4Science/dspace-angular/issues/43

Description

Short summary of changes (1-2 sentences).

• some additonal icon can be included in explore menus
• the icon can be configured in default-app.config.ts
• the icon works on internal routes as well as external links
• additional css classes can be configured in the icon to style the icon.

  Instructions for Reviewers

configure icon

Include guidance for how to test or review your PR. This may include: steps to reproduce a bug, screenshots or description of a new feature, or reasons behind specific changes.

Checklist

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• [ ] My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• [x] My PR passes ESLint validation using yarn lint
• [ ] My PR doesn't introduce circular dependencies (verified via yarn check-circ-deps)
• [ ] My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• [ ] My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• [ ] If my PR includes new libraries/dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• [ ] If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself.
• [x] If my PR fixes an issue ticket, I've linked them together.

[dominik-kern]

This is a nice feature, but how to add an icon to communities_and_collectionswhich is not an explore_section?