Add experiments watcher (webserver)

# Goals

Display a list of experiments on a webpage to easily check their status.
This webpage must be frequently updated, must be easy to customize, and must scale to 1000 experiments.

We would like to improve over this POC:

# Propositions

## Run the webserver

Using default options:
```bash
python -m bootstrap.watch --exp.dirs logs/myproject
```

Using custom options:
```bash
python -m bootstrap.watch -o myproject/options/watch.yaml
```

The yaml file `myproject_watch.yaml` is used to generate a custom webpage. It contains experiment directories to watch, filtering rules to select the experiment to display and columns to display.

Example:
```yaml
exp:
dirs: logs/myproject
filters:
- table: options
column: exp.dir
rule: mnist_resnet_*
- SQL: SELECT MAX(accuracy) FROM test_epoch WHERE accuracy > 0.3
columns:
- name: exp.dir
table: options
column: exp.dir
- name: accuracy
SQL: SELECT MAX(accuracy) FROM test_epoch WHERE accuracy > 0.3
- name: status
type: status
ranking:
- name: status
order: [crashed, ended, running]
- name: accuracy
order: asc
```

## Design of webpage

### Core features

Similarly to the POC, a first header with statistics of experiments followed by the custom table.

Example:
```
Ended: 10 | Crashed: 3 | Running: 40 (| Pending: 150)

server | exp.dir | accuracy | status
--- | --- | --- | ---
pascal[3] | lolilol | 0.19 | crashed
```

### Optional

In the header, we could have the list of filtering rules and columns to display. We could remove them or add new ones dynamically. Then, we could have a button to export as yaml file or update the original yaml file.

**Filtering rules**

Drop down pannel containing filtering rules which are based on SQLite data (select('options') or select('env_info') or select('train_epoch') or select('test_epoch') or custom SQL query).

Example:
```
List of positive filters:
- [table: options] [column: exp.dir] [rule: mnist_resnet_*]
- [table: env_info] [column: nodename] [rule: pascal & titan]
- [SQL: SELECT MAX(accuracy) FROM test_epoch WHERE accuracy > 0.3]
- [type: status] [rule: crashed]
- [info: end_datetime] [rule: >2020-05-22 10:00:00]
```
Display options that have a experiment directory which match the regexp `mnist_resnet_*`, are trained on `pascal`and `titan` server, have an accuracy higher than 0.3, and crashed after a certain datetime.

We could have a list of negative filters as well, corresponding to rules to remove experiments from the list.

**Column to display**

Same interface to select the column to display.
By default: server and gpu ids, experiment directory, number of epochs done / number of total epochs, datetime of creation, status.

Example:
```
... (default)
- [SQL: SELECT MAX(accuracy) FROM test_epoch]
```

## Implementation

```
bootstrap/watch.py
bootstrap/watch/css
bootstrap/watch/js
bootstrap/watch/index.html
```

Use [Werkzeug](https://github.com/pallets/werkzeug) to create webserver. (see [shortly example](https://github.com/pallets/werkzeug/tree/master/examples/shortly)).

Send in POST request all the options in the json format to the webpage.

Use simple javascript (no ReactJS) to get these options, look for experiment directories, send SQL queries to sqlite files, update the HTML. Every x seconds, update list of experiments and experiments if needed.