Add colocated media paths handling

[Spr-Aachen]

Issue

This is how my config looks like

media:
  - name: posts
    input: src/content/posts
    output: /src/content/posts

content:
  - name: posts
    path: src/content/posts
    fields:
      - { name: cover, type: image, options: {media: posts} }

The media files in my project stored in the same folder as the markdown content, which is a common pattern in Astro.
However, when I write cover: "./cover.jpg" in frontmatter (relative to the markdown file), the View on GitHub link in the editor incorrectly points to xxx/blob/main/cover.jpg instead of xxx/blob/main/src/content/posts/guide/cover.jpg.

Solution

Add mediaInput prop to FileTeaser and ImageTeaser components to handle relative media paths properly.
Comparing to the previous PR, this solution comes with minus changes to the code files.

I understand the documentation recommends to use a central folder, but many users prefer colocated media and it has been proved to work with Decap CMS. So I still hope this could be supported.