Allow specifying display names for servers

[towalink]

Allow specifying display names for servers. This addresses your comment from the previous pull request ("better representations on the UI").

The syntax follows what's known from email addresses (use of angle brackets).

Examples: ./frontend --servers="Gigs\<gigsgigscloud>,Hostdare\<hostdare>" --domain=dn42.lantian.pub ./frontend --servers="Prod\<prod.mydomain.local>,Test1\<fd88:dead:beef::1>,Test2\<fe80::c%wg0>" --domain=

Especially useful when servers are specified by IP addresses. But also to provide nicknames, etc.

Description of changes in the pull request:

• page.tpl: Use ServersDisplay variable for displaying the servers
• template.go: Revert previous change to map since insertion order is lost; beside server names keep escaped server names and server display names in the template
• render.go: Reflect change in template.go
• webserver.go: Use server display names for Bird template
• main.go: Parse display names and remember in settings struct
• render_test.go: Adapt test initialization to reflect changes
• README.md: Document new possibilities under "Advanced Features" (to keep the "quickstart" simple)

[xddxdd]

Thanks for your PR! Will merge this.