[dashboard][markdown] User can remove markdown component in dashboard view mode

[graceguo-supercat]

A clear and concise description of what the bug is.

How to reproduce the bug

1. create new dashboard,
2. add markdown component
3. save dashboard
4. Open dashboard, and hover on the markdown component
5. you can click trash icon to remove markdown component

Expected results

• Only in dashboard edit mode can change dashboard layout and components.
• Only dashboard owner can edit dashboard.

Screenshots

Environment

Lastest master

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

cc @junlincc @jinghua-qa @pkdotson