Use templ as template enine for dashboard component templates

[kron4eg]

What this PR does / why we need it:
This gives us nice type-safety in HTML template

Which issue(s) this PR fixes:

Fixes #

What type of PR is this?
/kind design

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

NONE

Documentation:

NONE

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[kubermatic-bot]

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