How do i install a webserver on my Turris Omnia (3.11.6)?

For uhttpd-tls, try this:

opkg update
wget http://archive.openwrt.org/chaos_calmer/15.05.1/mvebu/generic/packages/base/libustream-polarssl_2015-07-09-c2d73c22618e8ee444e8d346695eca908ecb72d3_mvebu.ipk -O /tmp/libustream-polarssl.ipk
opkg install /tmp/libustream-polarssl.ipk

and then continue:

opkg install uhttpd-mod-tls
opkg install luci-ssl

Unfortunately I can’t try it, I use TOS4 - another architecture.