Give `Link` input text field focus by default when creating hyperlinks for text in rich text editor

[cody_dunne]

The `Link` input field in the `Insert Link` modal dialog should be given focus by default when creating links from text within the rich text editor. This will reduce the number of steps required for adding hyperlinks.

Usage example: Copy URL from external resource with CTRL+C, select text in rich text editor, press CTRL+k to make it a hyperlink, then press CTRL+v to paste the copied URL. Currently, it requires an additional mouse click in the `Link` input field or pressing TAB twice.

If no text is selected: Still giving the `Link` field default focus would fit the model of copying and pasting a URL before writing the link text. The user could then press SHIFT+TAB to move focus back to the `Text` input field.

[ProductPanda]

Comments from Instructure

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