Release Joplin version 2.9.17

[mixin82]

Version 2.8.8 of the AUR package suffered from 2 issues:

• python2 was no longer available in Arch (see here). Release note for Joplin doesn't mention python2 either, but the generic python with resolves to 3.x.x in Debian derivatives (see here)
• build couldn't complete because of an incompatibility between "nan" package and new version of electron/node.js (see here and here)

The fix for nan package is present in 2.17.0, but Joplin 2.8.8 is hardcoded to use nan@2.15.0 for server package, hence the build failure.
In the meantime, Joplin version 2.9.17 has appeared, but unfortunately versions haven't been changed.
Good thing is that they updated versions in Joplin dev branch, so there's hope for the next version.
In my PR, I simply:

• bumped to Joplin version 2.9.17
• replaced python2 with python
• forced yarn to use nan@2.17.0 when resolving nan@^2.12.1

I didn't simply resolved nan to 2.17.0 because joplin-desktop requires version 2.14.2 and not ^2.14.2 (see here). It'd have probably been the same, but just to be safe...
The last change is temporary, so I conditionally added it based on the version.
Thanks