bgpd: Create BGP alias names for community/large-community

bgpd: Create BGP alias names for community/large-community

Show alias name instead of numerical value in `show bgp <prefix>. E.g.:

```
root@exit1-debian-9:~/frr# vtysh -c 'sh run' | grep 'bgp community alias'
bgp community alias 65001:123 community-1
bgp community alias 65001:123:1 lcommunity-1
root@exit1-debian-9:~/frr#
```

```
exit1-debian-9# sh ip bgp 172.16.16.1/32
BGP routing table entry for 172.16.16.1/32, version 21
Paths: (2 available, best #2, table default)
  Advertised to non peer-group peers:
  65030
    192.168.0.2 from home-spine1.donatas.net(192.168.0.2) (172.16.16.1)
      Origin incomplete, metric 0, valid, external, best (Neighbor IP)
      Community: 65001:12 65001:13 community-1 65001:65534
      Large Community: lcommunity-1 65001:123:2
      Last update: Fri Apr 16 12:51:27 2021
exit1-debian-9#
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>