ipsec: Add download button for swanctl.conf

[Monviech]

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Is your feature request related to a problem? Please describe.

For IPsec support, it would be quite beneficial to have the option to download the swanctl.conf from the GUI. Having that file removes any ambiguity like dealing with screenshots.

Optionally, there could be a diagnostics page where you could see the file in the GUI, with a download button beneath it.

The download button should maybe have a small help text or warning next to it that the PSKs are inside.

EDIT: There might be sanitation concerns for PSKs or Passwords. This has to be discussed more.