Can set couch_peruser.database_prefix with illegal characters

## Description

When using couch_peruser it is allowed to set database_prefix with illegal characters for a database name. For us this resulted in CouchDB crashing and being unable to start until we changed the configuration manually in local.ini

## Steps to Reproduce

1. Set up single node CouchDB
2. Run
```
curl -H 'Content-Type: application/json' -X PUT http://localhost:5984/_node/nonode@nohost/_config/couch_peruser/database_prefix -d '"userdb-.-"'
curl -H 'Content-Type: application/json' -X PUT http://localhost:5984/_node/nonode@nohost/_config/couch_peruser/enable -d '"true"'
```
3. CouchDB has now crashed, and will crash immediately on restart. The error messages I saw after attempted restart were:
```
Error: {error,{illegal_database_name,<<"userdb-.-">>}}
[error] 2019-10-31T14:28:37.769379Z nonode@nohost <0.416.0> -------- CRASH REPORT Process couch_peruser (<0.416.0>) with 0 neighbors exited with reason: bad return value {error,{illegal_database_name,<<"userdb-.-">>}} at gen_server:init_it/6(line:356) <= proc_lib:init_p_do_apply/3(line:247); initial_call: {couch_peruser,init,['Argument__1']}, ancestors: [couch_peruser_sup,<0.414.0>], messages: [], links: [<0.415.0>,<0.87.0>], dictionary: [], trap_exit: false, status: running, heap_size: 1598, stack_size: 27, reductions: 1586
[error] 2019-10-31T14:28:37.769649Z nonode@nohost <0.415.0> -------- Supervisor couch_peruser_sup had child couch_peruser started with couch_peruser:start_link() at undefined exit with reason bad return value {error,{illegal_database_name,<<"userdb-.-">>}} in context start_error
[error] 2019-10-31T14:28:37.770164Z nonode@nohost <0.413.0> -------- CRASH REPORT Process (<0.413.0>) with 0 neighbors exited with reason: {{shutdown,{failed_to_start_child,couch_peruser,{bad_return_value,{error,{illegal_database_name,<<"userdb-.-">>}}}}},{couch_peruser_app,start,[normal,[]]}} at application_master:init/4(line:134) <= proc_lib:init_p_do_apply/3(line:247); initial_call: {application_master,init,['Argument__1','Argument__2',...]}, ancestors: [<0.412.0>], messages: [{'EXIT',<0.414.0>,normal}], links: [<0.412.0>,<0.9.0>], dictionary: [], trap_exit: true, status: running, heap_size: 376, stack_size: 27, reductions: 105
[info] 2019-10-31T14:28:37.770459Z nonode@nohost <0.9.0> -------- Application couch_peruser exited with reason: {{shutdown,{failed_to_start_child,couch_peruser,{bad_return_value,{error,{illegal_database_name,<<"userdb-.-">>}}}}},{couch_peruser_app,start,[normal,[]]}}

```

## Expected Behaviour

Setting `couch_peruser.database_prefix` to a value with invalid database characters will throw an error.

## Your Environment

* CouchDB Version used: 2.3.1
* Operating System and version: Debain 10 (buster)

Start at the couch_peruser component and reproduce the configuration change using couch_peruser/database_prefix with the example invalid value. Trace its startup handling and add coverage for rejecting invalid database characters; done means the configuration reports an error without causing CouchDB to crash or fail to restart.

Written by the indexing model from the issue text.