Problem: calling .flatten() from itertools fails on nightly

```
   --> sit-web/src/webapp.rs:202:32
       |
   202 |             let record = match sit_core::repository::IssueRecordIter::flatten(issue.record_iter().unwrap()).find(|r| r.encoded_hash() == record) {
       |                                                                       ^^^^^^^ multiple `flatten` found
       |
       = note: candidate #1 is defined in an impl of the trait `std::iter::Iterator` for the type `sit_core::repository::IssueRecordIter<'_>`
       = note: candidate #2 is defined in an impl of the trait `itertools::Itertools` for the type `_`
       = note: candidate #3 is defined in the trait `rayon::iter::ParallelIterator`
```

This is, perhaps, related to the addition of the [`iterator_flatten`
nightly feature](rust-lang/rust#48213).

Solution: use itertools' version explicitly

Author: yrashk

sit-web/src/webapp.rs

@@ -199,7 +199,7 @@ pub fn start<A: ToSocketAddrs>(addr: A, config: sit_core::cfg::Configuration, re
                 Some(issue) => issue,
                 None => return Response::empty_404(),
             };
-            let record = match issue.record_iter().unwrap().flatten().find(|r| r.encoded_hash() == record) {
+            let record = match ::itertools::Itertools::flatten(issue.record_iter().unwrap()).find(|r| r.encoded_hash() == record) {
                Some(record) => record,
                None => return Response::empty_404(),
             };