log should better be shared as a file

[r10s]

with https://github.com/deltachat/deltachat-ios/pull/2126 we have the option to share a log, which is great!

however, the log is shared as text, which is unwieldy and error prone. we went through that on android recently :)

better share the log as a file, this allows the user also to take some notes - wrt editing: this is possible sufficiently beforehand