tiddly_on_tahoe header should link to immutable as well as to read-only links

[arch_o_median]

arc` clicked on the part of header that said "Click here for a read-only link to this page." and was later surprised that he had gotten a read-only mutable cap when he had assumed he had gotten an immutable cap. One good solution to this would be to offer both read-only-mutable and immutable links next to each other there.