Use links and buttons consistently

[arildm]

Use links for navigation, and buttons for actions.

Some buttons that should be links:

• "New corpus"
• "Edit" in the config panel
• "Manage access" in the sharing panel
• "Custom config" on the corpus config page

Proposed style guidelines below.

• Update code to match this
• Add this to dev docs

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• Use buttons for actions (that change app state or mental state) and links for navigation
  • If both state-changing and navigating, use a button
  • Remove link underline in places where links are default, e.g. app header
  • For buttons, use <RouteButton> or <ActionButton>, which use the mink-button class
    • Complement with button-{primary,danger,warning,success}, button-mute (less eye-catching) and button-slim (smaller)